Polyoxyalkylene polyether polyols are well known compounds. These polyether polyols are utilized, in conjunction with a cross-linking agent, such as an organic isocyanate, to form or produce a variety of polyurethane products, foamed and non-foamed, i.e., elastomeric, such as polyurethane foams and polyurethane elastomers. As a general matter, these polyols are produced by polyoxyalkylation of an initiator molecule with an alkylene oxide such as ethylene oxide, propylene oxide, butylene oxides, or mixtures thereof. The initiator molecules contain alkylene oxide-reactive hydrogens like those found in hydroxyl groups and amine groups. This oxyalkylation is generally conducted in the presence of a catalyst.
The most common catalysts are basic metal catalysts such as sodium hydroxide, potassium hydroxide, or alkali metal alkoxides. One advantage of these basic metal catalysts is that they are inexpensive and readily available. Use of these basic metal catalysts, however, is associated with a range of problems. One of the major problems is that oxyalkylation with propylene oxide has associated with it a competing rearrangement of the propylene oxide into allyl alcohol, which continually introduces a monohydroxyl-functional molecule. This monohydroxyl-functional molecule is also capable of being oxyalkylated. In addition, it can act as a chain terminator during the reaction with isocyanates to produce the final polyurethane product. Thus, as the oxyalkylation reaction is continued more of this unwanted product, generally measured as the unsaturation content of the polyol, is formed. This leads to reduced functionality and a broadening of the molecular weight distribution of the polyol. The amount of unsaturation content may approach 30 to 40 molar % with unsaturation levels of 0.090 meq KOH/g or higher.
In an attempt to reduce the unsaturation content of polyether polyols, a number of other catalysts have been developed. One such group of catalysts includes the hydroxides formed from rubidium, cesium, barium, and strontium. These catalysts also present a number of problems. The catalysts only slightly reduce the degree of unsaturation, are much more expensive, and some are toxic.
A further line of catalyst development for polyether polyol production focuses on double metal cyanide (DMC) catalysts. These catalysts are typically based on zinc hexacyanocobaltate. With the use of DMC catalysts, it is possible to achieve relatively low unsaturation content in the range of 0.003 to 0.010 meq KOH/g. While the DMC catalysts would seem to be highly beneficial they also are associated with a number of difficulties. As a first difficulty, there is a relatively high capital cost involved in scaling up of and utilization of DMC catalysts. The catalysts themselves have an extremely high cost compared to the basic metal catalysts. Further, when forming a polyether polyol using a DMC catalyst, there is a significant initial lag time before the DMC catalyst begins to catalyze the reaction. It is not possible to add ethylene oxide onto growing polyol chains utilizing DMC catalysts. To add ethylene oxide to a growing chain, the DMC catalysts must be replaced with the typical basic metal catalysts, thus adding complexity and steps. In addition, it is generally believed that the DMC catalysts should be removed prior to work-up of any polyether polyol for use in forming polyurethane products. Finally, polyether polyols generated using DMC catalysts are not mere “drop in” replacements for similar size and functionality polyols produced using the typical basic metal catalysts. Indeed, it has been found that often DMC catalyzed polyether polyols have properties very different from equivalent polyether polyols produced using, for example, potassium hydroxide.
More recent lines of catalyst development for polyether polyol production focus on aluminum phosphate and aluminum phosphonate catalysts. However, these catalysts also have drawbacks. Both aluminum phosphate catalysts and aluminum phosphonate catalysts may be subject to slow hydrolysis upon exposure to water, such as the water present in the air as humidity or even water present as an impurity in the initiator molecule and alkylene oxide reactants.
Finally, it is known that simple carboxy-modified aluminum compounds, i.e., those not including phosphate and/or phosphonate, are not catalytically active and are, therefore, not useful for the formation of polyether polyols.
Thus, there exists a need for a class of catalysts that can be used for the oxyalkylation of initiator molecules by alkylene oxides that are inexpensive, capable of producing very low unsaturation polyether polyols, do not require removal from the polyether polyol prior to utilization to form a polyurethane product, and that produce a polyether polyol having properties that are the same or better than those in polyether polyols produced using basic metal catalysts. The need also extends to a class of catalysts having improved stability as determined by resistance to hydrolysis upon exposure to water. It would also be beneficial if the new class of catalysts could be used in existing systems and equipment using standard manufacturing conditions.